背景
TraceId能标记一次请求的调用链路，在我们排查问题的时候十分重要。系统引入MQ后，MQ消息默认不带TraceId，所以消息发送和处理的链路就断了。下面分享如何对业务逻辑无感的方式，将TraceId带到消费端。
难点
RabbitMQ的Message对象可以在属性上设置头信息，所以携带TraceId的位置有了，问题是怎么无感的方式设置和获取TraceId？
Spring RabbitMQ拦截器
在Spring里使用RabbitMQ本身没有拦截器，但是有一个消息处理器，可以在发送和接收消息之前对消息进行处理。里面有3个重载的方法，对原始消息进行转换。我们可以借助这个处理器，在Message对象里加上TraceId。
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| public interface MessagePostProcessor { /** * Change (or replace) the message. * @param message the message. * @return the message. * @throws AmqpException an exception. */ Message postProcessMessage(Message message) throws AmqpException; /** * Change (or replace) the message and/or change its correlation data. Only applies to * outbound messages. * @param message the message. * @param correlation the correlation data. * @return the message. * @since 1.6.7 */ default Message postProcessMessage(Message message, Correlation correlation) { return postProcessMessage(message); } /** * Change (or replace) the message and/or change its correlation data. Only applies to * outbound messages. * @param message the message. * @param correlation the correlation data. * @param exchange the exchange to which the message is to be sent. * @param routingKey the routing key. * @return the message. * @since 2.3.4 */ default Message postProcessMessage(Message message, Correlation correlation, String exchange, String routingKey) { return postProcessMessage(message, correlation); } } |
发送消息时，携带TraceId
Spring默认使用RabbitTemplate来发送消息，RabbitTemplate的send方法在发送消息之前，会调用beforePublishPostProcessors来处理Message对象。beforePublishPostProcessors集合里存的就是MessagePostProcessor对象，它会在发消息之前执行：
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| public void doSend(Channel channel, String exchangeArg, String routingKeyArg, Message message, boolean mandatory, @Nullable CorrelationData correlationData) throws IOException { // ... // 核心代码 if (this.beforePublishPostProcessors != null) { for (MessagePostProcessor processor : this.beforePublishPostProcessors) { messageToUse = processor.postProcessMessage(messageToUse, correlationData, exch, rKey); } } // ... sendToRabbit(channel, exch, rKey, mandatory, messageToUse); // ... } |
我们要做的是在注册RabbitTemplate的时候加上处理TraceId的逻辑
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| @Bean public RabbitTemplate rabbitTemplate(RabbitTemplateConfigurer configurer, ConnectionFactory connectionFactory) { RabbitTemplate template = new RabbitTemplate(); configurer.configure(template, connectionFactory); template.setBeforePublishPostProcessors(new MessagePostProcessor() { @Override public Message postProcessMessage(Message message) throws AmqpException { // 这个方法没调用，调用的是下面那个方法 return message; } @Override public Message postProcessMessage(Message message, Correlation correlation, String exchange, String routingKey) { String requestId = MDC.get(MdcConstants.REQUESTID); if (StringUtils.isEmpty(requestId)) { requestId = StringUtils.random(); } message.getMessageProperties().setHeader(RabbitMQConstants.HEADER_REQUESTID, requestId); return message; } }); return template; } |
消费消息时，获取TraceId
当我们通过@RabbitListener注册consumer时，Spring会通过org.springframework.amqp.rabbit.listener.SimpleMessageListenerContainer.AsyncMessageProcessingConsumer#mainLoop方法，不断从consumer队列里拿到消息。在把消息交给@RabbitListener标注的对象前，也会对Message对象进行处理。这里的处理器是存在org.springframework.amqp.rabbit.listener.SimpleMessageListenerContainer#afterReceivePostProcessors属性上。
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| private void doExecuteListener(Channel channel, Object data) { if (data instanceof Message) { Message message = (Message) data; if (this.afterReceivePostProcessors != null) { for (MessagePostProcessor processor : this.afterReceivePostProcessors) { message = processor.postProcessMessage(message); if (message == null) { throw new ImmediateAcknowledgeAmqpException( "Message Post Processor returned 'null', discarding message"); } } } // ... invokeListener(channel, message); } else { invokeListener(channel, data); } } |
怎么设置SimpleMessageListenerContainer#afterReceivePostProcessors的值？
SimpleMessageListenerContainer对象是由SimpleRabbitListenerContainerFactory工厂对象创建，在创建SimpleMessageListenerContainer对象时，会把工厂里的属性拷贝过来，afterReceivePostProcessors就是通过工厂拷过来。所以我们直接设置SimpleRabbitListenerContainerFactory的afterReceivePostProcessors值就可以。
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| @Bean(name = {"rabbitListenerContainerFactory"}) @ConditionalOnProperty( prefix = "spring.rabbitmq.listener", name = {"type"}, havingValue = "simple", matchIfMissing = true ) public SimpleRabbitListenerContainerFactory simpleRabbitListenerContainerFactory(SimpleRabbitListenerContainerFactoryConfigurer configurer, ConnectionFactory connectionFactory, ObjectProvider<ContainerCustomizer<SimpleMessageListenerContainer>> simpleContainerCustomizer) { SimpleRabbitListenerContainerFactory factory = new SimpleRabbitListenerContainerFactory(); configurer.configure(factory, connectionFactory); simpleContainerCustomizer.ifUnique(factory::setContainerCustomizer); factory.setAfterReceivePostProcessors(message -> { Object requestId = message.getMessageProperties().getHeader(RabbitMQConstants.HEADER_REQUESTID); if (StringUtils.isEmpty(requestId)) { requestId = StringUtils.random(); } MDC.put(MdcConstants.REQUESTID, String.valueOf(requestId)); return message; }); return factory; } |
这样，我们就能在@RabbitListener的消费逻辑里拿到TraceId。